NACs are a critical part of alarm systems, for example, fire alarm systems, because they provide power and control to notification appliances, such as bells, horns, strobes, and the like, that must operate when evacuation from a region in alarm is necessary. Because of the critical nature of NACs, NACs must be supervised, that is, monitored for wiring faults.
Known NAC supervision schemes require EOL resistors at a far end of the NAC, and the value of such EOL resistors is often limited to a predetermined value, which can vary considerably from one model of a control panel to another. The value of an EOL resistor is critical because an incorrect value may cause supervision current to fall outside of a predetermined normal range.
For example, known NACs can be supervised by running a small current through the wiring of the NAC and the EOL resistor in a reverse direction. That is, when notification appliances are activated by current flowing from a positive terminal to a negative terminal, supervision current can flow from the negative terminal to the positive terminal. The supervision current can be measured, for example, with a resistor in series that develops a corresponding voltage that can be compared to high and low limits. The measured supervision current should fall within a predetermined normal range. Indeed, if the wiring is short circuited, the supervision current will be above the normal range. Similarly, if the wiring is open circuited, the supervision current will be below the normal range.
When replacing a control panel, such as a fire alarm control panel (FACP) in an existing building, that is, when retrofitting a control panel, problems can arise. For example, it can be difficult to locate and replace an existing EOL resistor when new equipment requires an EOL resistor with a different value. Some NACs address this problem by tuning the circuit, but the user must set or install a reference resistor that matches the existing EOL resistor. Accordingly, tuning the NAC can be a time consuming and inconvenient process.
Furthermore, many known NACs make a distinction only between the presence and absence of an EOL resistor and assume that an EOL resistor with a proper value is present. However, if an EOL resistor with an incorrect value is present, supervision may not function correctly. For example, a functioning circuit may be reported as faulty or vice versa. Additionally, the value of the EOL resistor may not be reported to a user because the resistance of the EOL cannot be accurately measured.
In addition to the above, when installing a control panel, for example, a FACP, or related equipment, it is often difficult to find and repair ground faults. Indeed, finding and repairing ground faults is often a time consuming process of troubleshooting to localize the fault to one particular circuit, and this can result in wasted time and extra expense. Some approaches have been developed to reduce the troubleshooting time, but these known solutions often require the expensive galvanic isolation of each NAC in a system.
In view of the above, there is a continuing, ongoing need for improved systems and methods.